# Potassium ferrocyanide is used in the detection of

(a) $Cu^{2+}$ ions
(b) $Fe^{3+}$ ions
(c) Both $Cu^{2+}$ & $Fe^{3+}$ ions
(d) None

Answer: Both $Cu^{2+}$ & $Fe^{3+}$ ions

Both $Cu^{2+}$ & $Fe^{3+}$ ions can be detected with potassium ferrocyanide.
$2CuSO_4 + K_4[Fe(CN)_6] \longrightarrow Cu_2[Fe(CN)_6]$ (Chocolate ppt.) $+ 2K_2SO_4$
$4FeCl_3 + 3K_4[Fe(CN)_6] \longrightarrow Fe_4[Fe(CN)_6] \text{{ferriferrocyanide (prussian blue)}} + 12KCl$